FIG. 9 shows a wireless communication apparatus to which an automatic gain controller (AGC) according to the related art is applied. In the wireless communication apparatus, a signal received by an antenna 2 is amplified by an amplified 3 and a variable gain amplifier 4. An I-component and a Q-component of the amplified signal are isolated by a mixer 5. The isolated signals are then subject to analog-to-digital conversion by analog-to-digital converters 6i and 6q, respectively and are decoded by a processing unit 7. An automatic gain controller 8 calculates a difference between an output value of the analog-to-digital converter 6i and a reference value and also calculates a difference between an output value of the analog-to-digital converter 6q and the reference value so as to produce an error signal. The automatic gain controller 8 compares the error signal with a threshold value and so as to update the gain of the variable gain amplifier 4 at predetermined intervals in accordance with a result of comparison. With this construction, signals at a level adapted to the dynamic range of the analog-to-digital converters 6i and 6q are input to the analog-to-digital converters 6i and 6q so that the precision in conversion by the analog-to-digital converters 6i and 6q is maintained at a proper level.
For example, published Japanese translation of PCT international publication for patent application No. 10-506764 discloses automatic gain control in which the gain is controlled as a result of performing comparison between the error signal and the threshold value.
In the related art, however, the gain is updated only after a predetermined period of time elapses even when the environment for reception changes as a result of, for example, a rapid variation in the level of received power. For this reason, a signal not adapted for the dynamic range of the analog-to-digital converter may be input to the analog-to-digital converter when there is a rapid change in the level of received power, resulting in decrease in the precision in conversion by the analog-to-digital converter.